Optical pointing devices, such as optical mice, direct movement information to a graphical user interface of a computer by tracking relative movement between a navigation surface, such as a mouse pad, and an image sensor within the optical pointing devices. Light is optically radiated onto the navigation surface by a light source, such as a light emitting diode or a laser. Based on the illumination of the navigation surface, images are obtained using electronic image sensors, which typically contain an array of photodetectors arranged in a pattern. Each photodetector in the array creates an output signal proportional to the intensity of light impinging on that photodetector. The output signals from the photodetector array are processed to generate an image of the navigation surface. By comparing a series of these images over time, the optical mouse develops movement information about movement of the mouse relative to the navigation surface. This movement information, upon processing, enables a corresponding movement of a pointer on the graphical user interface of the computer.
Some optical pointing devices use a coherent light source (e.g., a laser) to illuminate the navigation surface. While coherent light sources enable lower power consumption and more accurate and precise optical navigation, these coherent illumination navigation systems are more sensitive to the presence of contaminants, such as particles of dust, dirt, food, hair, and other substances. In contrast, optical pointing devices that use a non-coherent light source (e.g., a light emitting diode) are relatively unaffected by contaminants such as dirt and dust because they employ diffuse light patterns which cause the contaminants to remain out of focus in the images.
However, for coherently illuminated pointing devices, once a contaminant becomes fixed to one of the optical surfaces of the optical pointing device (e.g., an imaging lens, illumination lens, etc) and is coherently-illuminated, it becomes a fixed pattern on the image of the navigation surface. For example, with a contaminant present, one or more photodetectors of the photodetector array of the image sensor will generate a fixed output signal having an intensity or location, that does not correspond to an actual surface feature or light pattern on the navigation surface. This fixed pattern acts as noise, distorting image comparison and thereby inhibiting accurate tracking of movement of the optical pointing device relative to the navigation surface. Unfortunately, contaminants are unavoidable during transport or use in dusty environments.
Accordingly, while coherent light sources hold the promise of improved optical navigation for optical pointing devices, there is a need to improve the contaminant resistance of coherently illuminated pointing devices.